Ryohei Morimoto

Types of volcanic eruptions in Japan

7. Summary and conclusionThe eruptions of volcanoes are various in surface features as well as in magnitude, depending upon their geographical position, structure, crater condition, physical and petrochemical nature of the ejecta, etc. Referring principally to the recent eruptions which occurred within the last few scores of years in Japan and which have been described in detail by the investigators, these active volcanoes in Japan are classified as shown in the tables (Tables 1 and 2). First of all, the active volcanoes are divided into two groups according to the frequency of eruption. One includes those volcanoes which are literally active with eruptions every few years; they usually have a persistent open crater from which the eruptions are taking place. Another includes those volcanoes which are usually dormant and are awakened into an eruption once in more than several scores of years; they usually have a closed crater and their next eruptions are liable to occur at new craters more or less inconsistent with the craters of the preceding eruptions. Eruptions themselves are divided first into magmatic and phreatic eruptions. The magmatic eruptions are then divided into effusive, mixed, and explosive eruptions. The effusive eruptions are rather simple, while the mixed and explosive eruptions may be further divided into certain sub-types according to the mode of occurrence of the ejecta.As a means of quantitatively specifying the magnitude of volcanic eruptions, these are classified into ten grades (O to IX), according to the volume of solid ejecta, on the assumption that the intensity of eruption is proportional to the total volume of ejecta. This scale (Table 3) is applied to the eruptions, the volume of whose ejecta have been estimated.

Low temperature mud-explosion of Mt. Yaké, Prefs. Nagano-Gifu, Central Japan, on 17th June 1962 as an example of endogenous katamorphism of volcanic rock at the destructive stage of the volcano

Mt. Yaké or Yaké-daké is a dissected dome-shaped volcano mainly composed of the biotite bearing augite-hypersthene-hornblende andesite lavas extruded on the high mountain ridge consisting of the granite and hard Palaeozoic rocks between two prefectures Nagano and Gifu in the central part of Japan. It had been almost in dormant state only with weak fumarole activity on and around its summit dome since the former active period from 1907 to 1932. Incandescent lava emission has never been recorded in the historic age.On 17th June 1962 at about 21 h 55 m, a sudden explosion took place on the northern side of the dome. After successive explosions a fissure, about 700 m in length, was formed. On 19th from the northeast end of the fissure, milky hot water suspending muddy material flowed out. The mud flow ran down on the slope along the dry gully and poured into the Lake Taisyo-iké, about 2.5 km east of the vent. The lake was formed in 1915-eruption when a tremendous mud flow dammed up Azusagawa, the river running through the valley east of the volcano.Ejected blocks were deposited on the area within 1 km from the vent. Ash was deposited about 1 cm in thickness on the area about 4 km east of the volcano. Several mud flows poured into the Lake Taisyo-iké and the River Azusagawa. But no red-hot ejecta was observed during the present eruption, and temperature near the vent was lower than 100°C. Thus the present eruption is said to be low temperature phreatic explosions.In suspensoids of the hot water and in clayey matter deposited around the new vent are contained the montmorillonites, which hove never been found in the rocks exposed on the volcano in spite of the detailed investigation of the writers over 10 years. On the other hand, the mineral is not expected to be formed in the altered rocks under oxydized state on the surface.It was fine, at least no rain, before and during the explosions and the mud flow ran down along the dry gully. So the hot water was purely derived from the inner part of the volcano and the mud flow was not brought about by rain fall after deposition of ejecta on the volcano.The mud flow must have been formed endogenously under the volcano where the katamorphism of the rocks forming the volcano had advanced owing to chemical action of volcanic gas in the long period before the eruption.